The present invention relates to a method and apparatus for locating an object that provides identifying signals.
There are several techniques which can be utilised to locate the position of an object relative to a reference point, for example, the position of an aircraft with respect to one of a plurality of sites. One such technique is described in GB-B-2 250 154.
In GB-B-2 250 154, an object location system is described in which a master or reference receiver and a plurality of auxiliary receivers are precisely located with respect to one another for receiving signals from an object. The signals received at the receivers from the object comprise secondary surveillance radar (SSR) generated by the object. Each auxiliary receiver is synchronised with respect to a signal sent from the reference receiver so that a computation device associated with the reference receiver can determine the precise location of the object from signals received at all the receivers. In this system, the reference receiver and at least three other auxiliary receivers are required to generate four simultaneous equations which are solved by the computation device to provide the position of the object in three dimensions and the distance the object is from the reference receiver.
SSR is also used to effect synchronisation between the reference receiver site and the auxiliary receiver sites. Each auxiliary receiver site utilises an atomic clock for driving a free running counter which is read on reception of an SSR signal transmitted thereto from the reference receiver site. However, in the object location system described in GB-B-2 250 154, it is necessary for each auxiliary receiver site to have a direct xe2x80x98line of sightxe2x80x99 with the master or reference receiver""site so that the timing signals can be received.
Another technique for providing synchronisation between the reference receiver and the auxiliary receivers disclosed in GB-B-2 250 154 utilises the global positioning system (GPS), GPS being used at each receiver site to provide both accurate location and intersite clock synchronisation.
It is therefore an object of the present invention to provide an object location system which does not have the disadvantages mentioned above.
In accordance with one aspect of the present invention, there is provided a method of locating the position of an object which provides identifying signals using a receiver array comprising a reference receiver site and at least three auxiliary receiver sites which are not on a line of sight with the reference receiver site, the method comprising the steps of:
receiving the identifying signals from the object at each receiver site;
receiving signals from a global navigation system at each receiver site and using the received signals for providing timing signals;
providing a clock at each receiver site for providing clock signals;
combining the timing signals with the clock signals to provide a timing reference signal at each receiver site;
determining the time difference between receiving the identifying signals and the timing reference signal at each receiver site;
transmitting data signals to the reference receiver site which are indicative of the determined time differences at each auxiliary receiver site; and
calculating the position of the object using the transmitted data signals and the time difference determined at the reference receiver site.
By the term xe2x80x98line of sightxe2x80x99 is meant a direct line between the two points without the presence of any obstacles.
In accordance with another aspect of the present invention, there is provided apparatus for locating an object producing identifying signals, the apparatus comprising:
a receiver array comprising a reference receiver site and at least three auxiliary receiver sites which are not on a line of sight with the reference receiver site;
first receiver means at each receiver site for receiving the identifying signals from the object;
second receiver means at each receiver site for receiving timing signals from a global navigation system;
clock means at each receiver site for generating clock signals;
combining means at each receiver site for combining the timing signals and clock signals to provide a timing reference signal;
processing means at each receiver site for receiving signals from the first receiver means and the combining means and for providing a time difference measurement from the received signals; and
control means at the reference receiver site for receiving data signals from the auxiliary receiver sites indicative of the time difference measurement signals determined thereat, and for processing the data signals together with the time difference measurement from the reference receiver site to provide a determination of the location of the object.
Other objects, advantages and novel features of the present invention, will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.